The present invention relates to detection and identification of chemicals and, more particularly, to a method, device and system for detecting and identifying low levels of chemical agents, warfare chemical agents in particular.
Detection and identification of chemical agents include, inter alia, the use of surface acoustic wave detectors, ion mobility spectrometers, flame photometric detectors and the like.
In surface acoustic wave detectors, the target chemicals are absorbed or adsorbed onto a specific coating of a piezoelectric substrate, to thereby vary its mass. The mass change affects the resonance frequency of the piezoelectric substrate which is measured using an appropriate electronic circuitry.
In ion mobility spectrometer, a gaseous sample is ionized in an ionization region within the spectrometer, e.g., using a radioactive source, and accelerated over a short distance to a detector. The gaseous sample is analyzed by measuring a characteristic time-of-flight of the negative and positive ions from the ionization region to the detector.
In flame photometric detectors (FPDs) a gaseous sample is introduced to a hydrogen rich flame and electrons in the outer shell of atoms obtained from the target chemicals are excited to higher energy states. When an excited electron returns to its ground state, energy is emitted in the form of light by which the presence of target chemicals is confirmed. The wavelength of the emitted light depends on the target chemical, whereas its intensity depends on the chemical's concentration.
The above techniques have limited sensitivity and selectivity, in particular in environments, such as battlefield or industrial environments, in which the detection and identification of chemical agents must be performed under less than optimal conditions. It is recognized that when the chemical agents are extremely toxic, for example in the case of chemical warfare agents, very low levels thereof must be detected rapidly and efficiently.
Reliable on-site, real time, detection of trace levels of chemical agents is of utmost importance in particular for highly toxic chemical agents where lack of sensitive and accurate identification can dramatically increase the number of casualties. To prevent injury resulting from exposure to toxic chemicals, the presence of toxic chemicals must be detected while their concentrations are below toxic levels. Accordingly, to detect highly toxic chemicals, devices capable of detecting and identifying low concentrations within a short period of time are needed. For example, the toxic threshold level values of O-Ethyl S-2-diisopropylaminoethyl methyl phosphonothiolate (VX) and O-isopropyl methyl phosphonofluoridate (sarin—GB) are, respectively, 1×10−5 and 1×10−4 μg/L [Department of Defense (DOD) ammunition and explosive safety and standards, 1992, assistant secretary of defense (production and logistics, October 1992, DOD 6055.9 STD]. These values are about two orders of magnitude lower than the toxic threshold level value of common pesticides, e.g., parathion [Niosh pocket guide to chemical hazards, www.cdc.gov/niosh/npg].
Portable detectors based on the above techniques are known (to this end see, e.g., Brletich N. R., Waters M. J., Bowen G. W., Tracy M. F., “Worldwide Chemical Detection Equipment Handbook,” CBIAC, October 1995). However, when these devices are used in the field, their performance is often compromised, e.g., due to lack of supportive periphery. For example, detection limits of conventional hand-held chemical warfare agent detectors are from about 10−2 to about 10−1 μg/L [N. R. Brletich, M. J. Waters, G. W. Bowen and M. F. Tracy, “Worldwide Chemical Detection Equipment Handbook,” CBIAC, October 1995], which is about two or three orders of magnitude higher than the toxic threshold level values of VX and sarin as well as other hazardous chemical agents.
Several laboratory devices were adapted for field application in the past [U.S. Pat. No. 5,611,846; H. L. Meuzelaar, J. P. Dworzanski, N. S. Arnold, W. H. McClennen and D. J. Wager, Field Anal. Chem. Tech., 4, 3 (2000); E. R. Badman and R. G. Cooks, J. Mass Spectrom., 35, 659 (2000); and J. A. Syage, M. A. Hanning-Lee and K. A. Hanold, Field Anal. Chem. Tech., 4, 204 (2000)].
These devices, however, are expensive and are not sufficiently robust for massive deployment in the field.
Furthermore, due the continuously increasing demands of the modern battlefield environment, the required detection sensitivity of hand-held detectors is likely to be significantly increased, far beyond the capabilities of conventional devices.
One method of improving conventional chemical warfare agent detectors is disclosed in U.S. Pat. No. 6,455,003. In this method, the sample is enriched, prior to detection by the detector, by collecting a portion of the target chemicals within a sorbent element. Subsequently, the target chemicals are thermally desorbed into the detector [J. M. Sanchez and R. D. Sacks, Anal. Chem., 75, 978 (2003)].
An injection assembly for short path thermal desorption apparatus is disclosed in U.S. Pat. No. 5,123,276. The injection assembly includes a desorption tube for collecting and storing the sample compound to be analyzed and a needle injector for passing the desorbed sample component to a gas chromatograph unit for identification and quantification of the sample component.
U.S. Pat. No. 6,477,905 discloses a device for measurement of organic compound contaminants in a fluid sample stream. The device includes adsorbent trap for adsorbing the organic compound contaminants, while venting out permanent gases. The adsorbent trap is capable of rapid heating and cooling for rapidly desorbing the organic compound contaminants therefrom. Once desorbed, the contaminants enter a detector for measurement and analysis.
Sampling units are commercially available from CMS Research Corporation, Birmingham, Ala. or CDS Analytical, Inc., Oxford, Pa. These units are capable of detecting or improving detection capability of existing chemical agent detectors [see, e.g., U.S. Pat. Nos. 4,180,389 and 5,014,541]. Yet, their operation requires pure compressed gases and other consumable items, which makes massive deployment in the field problematic.
Additional prior art of interest is Amirav et al. [A. Amirav and G. Frishman, Field Anal. Chem. Tech., 4, 170 (2000)] in which low levels of chemical warfare agent simulants (stable, non toxic organo phosphor/sulfur compounds) were separated with a fast Gas Chromatograph (GC) system, equipped with a Pulsed Flame Photometric Detector (PFPD).
The use of air as a carrier gas for gas chromatographic separations has been investigated [A. J. Grall and R. D. Sacks, Anal. Chem., 71, 5199 (1999)] by separating stable volatile organic molecules with a laboratory gas chromatograph, using air as a carrier gas.
U.S. Pat. No. 6,223,584 discloses a system having an analyzing gas chromatographic unit, an in-line pre-concentrator assembly, an adsorbent material and a transfer line unit. When a trap housing present in the pre-concentrator assembly is displaced from the transfer line unit, the medium surrounding the trap housing is forced inside of the trap housing and vapor constituents are adsorbed on the adsorbent material. When the trap housing is moved with the transfer line unit, the adsorbent material is heated to release the vapor constituents from the adsorbent material.
Still additional prior art of relevance include U.S. Pat. Nos. 3,159,996, 4,420,679, 5,014,541, 5,005,399, 5,782,964, 5,665,314, 5,830,353, 6,093,921, 6,209,386, 6,217,829 and 6,530,260, the contents of which are hereby incorporated by reference.
However, the above attempts present several difficulties and limitations, especially in conjunction with portable detectors.
First, as the sensitivity enhancement is achieved by sorbing the target chemicals from a large volume of air and desobing it into a smaller volume, a skilled artisan would appreciate that portable detectors should, in principle, pump large volumes of air (typically about 0.5-5 L/min). Therefore, in order to obtain a gain factor of, say, 100, the sample volume should be about 50-500 L. Sampling such large volumes of air is both time consuming and requires large amount of power.
Second, in known sample enrichment methods, the chemical warfare agents, like any other semi volatile organic compound, desorb slowly from the sorbent material. Slow desorbtion dilutes the sample and reduces the sensitivity and selectivity of the detection process. In laboratory devices the slow desorbtion problem can be resolved by utilizing cryofocusing prior to detection. In portable devices, in contrast, cryofocusing is not applicable.
Third, in known sample enrichment methods the chemical warfare agents which are thermo-labile target chemicals may decompose during the thermal desorbtion.
Forth, it is difficult to operate prior art systems employing sample enrichment units in open field, because in such conditions the sorbent material tends to degrade, hence to decrease the efficiency of the detection and identification process.
There is thus a widely recognized need for, and it would be highly advantageous to have a method, device and system for detecting and identifying chemical agents, devoid of the above limitations.